1. Field of the Invention
The present invention relates to an apparatus and a method for controlling an irradiation direction of a vehicle lamp such as a headlamp and, in particular, relates to an apparatus and method for controlling the irradiation direction of a headlamp in a vertical direction in accordance with an angle of the front portion of a vehicle in a vertical direction with respect to a surface of a road.
2. Related Art
An amount of deflection of the spring of a suspension system which supports a vehicle body changes in accordance with changes of the loading state of goods or difference of passengers etc. within the vehicle. When the amount of deflection of the spring changes, a vertical depression angle of a front portion of the vehicle body with respect to a road surface, that is, an angle in a vertical direction (hereinafter called a pitch angle in this specification) the vehicle body along a longitudinal direction thereof with respect to the road surface may change. Thus, an irradiation direction (optical axis) of the headlamp attached to the front portion of the vehicle also changes in the vertical direction so as to follow the pitch angle. As a result, drivers of other vehicles such as oncoming vehicles are dazzled when the irradiation direction is directed upward, whilst an amount of light irradiation in the forward direction of the vehicle becomes insufficient when the irradiation direction is directed downward, both of which are not preferable in a view point of traffic safety. In order to keep the irradiation direction of the headlamp irrespective of such a change of the pitch angle of a vehicle, there is an auto-leveling apparatus. The auto-leveling apparatus includes a pitch angle detection means for detecting the pitch angle of the vehicle, a control means for outputting a control signal for adjusting the irradiation angle of the headlamp in the vertical direction in accordance with the pitch angle thus detected, and an actuator for adjusting the angle of the headlamp in the vertical direction with respect to the vehicle body based on the control signal from the control means.
According to such an auto-leveling apparatus, in most cases, the actuator and the headlamp are integrally constituted and the pitch angle detection means is constituted as a part of the controller of a vehicle together with the control means. Thus, the controller including the pitch angle detection means and the control means is coupled to the actuator by means of wire harnesses. When a failure occurs in the actuator, the controller recognizes the failure and is required to perform a so-called fail-safe operation such as the reduction of the light intensity of the headlamp or the fixing of the irradiation direction of the headlamp to a predetermined position. To this end, the controller is required to grasp the failure of the actuator. Disclosed in JP-A-2002-036945 is a technique in which a power supply voltage supplied to the actuator is monitored, and the actuator is stopped at the current position when the power supply voltage changes by a reference value or more. Disclosed in JP-A-2004-155249 is a technique in which a detection circuit is provided for detecting a current flowing through the coil of a motor constituting the actuator, and the failure diagnosis of the motor is performed based on the detection data of the detection circuit at the time of flowing a failure diagnosis current and at the time of not flowing the failure diagnosis current.
However, in the conventional techniques, in order for the controller to determine the abnormality of the actuator, it is required to connect the actuator with the controller by means of wire harnesses independently provided for determining the abnormality. As a result, there arise such problems that the number of cables increases, the number of the terminals of the controller increases in particular, which results in the difficulty in miniaturizing and light-weighting the apparatus and further reducing the cost of the apparatus. In JP-A-2002-036945, a monitor circuit independently provided for monitoring the abnormality of the power supply voltage and the wire harness for monitoring are required. In JP-A-2004-155249, the wire harness for flowing the current for the failure diagnosis and the detection circuit is required. Since each of JP-A-2002-036945 and JP-A-2004-155249 detects the abnormality of the actuator itself, when the wire harness for transmitting the control signal among the wire harnesses for connecting the controller and the actuators is broken or short-circuited, a normal control signal can not be sent to the actuator any more. In this case, since there arises no abnormality in the actuator itself, the abnormality of the control signal line can not be detected quickly.